1. Technical Field of the Invention
The present invention relates generally to an oscillatory angular rate sensor, and more particularly to an improved structure of such an angular rate sensor designed to minimize an output error.
2. Background Art
Typical oscillatory angular rate sensors have a sensor element equipped with an oscillator. The sensor element includes a substrate on which the oscillator is installed, an oscillation exciting mechanism, and an angular velocity sensing mechanism. The oscillator is supported by beams so that it may move elastically in a first direction and second direction oriented perpendicular to each other and in parallel to the substrate. The oscillation exciting mechanism excites the oscillator to oscillate in the first direction. The angular velocity sensing mechanism works to sense oscillatory movement of the oscillator in the second direction arising from angular motion of the sensor to determine the angular velocity of the sensor.
When the whole of the sensor experiences angular motion about an axis (will be referred to as an angular velocity sensing axis below) extending in a third direction perpendicular to the substrate during oscillation of the oscillator in the first direction parallel to the substrate, it will cause the Coriolis force to be produced which oscillates the oscillator in the second direction perpendicular to the first direction. The angular velocity sensing mechanism senses the degree of such an oscillation to determine the angular velocity the sensor is undergoing.
For example, Japanese Patent First Publication Nos. 2000-28365 and 5-312576 disclose angular rate sensors of the above type.
The above described angular rate sensor is so designed as to allow the oscillator to also oscillate in the second direction when the acceleration is applied externally to the sensor in the second direction. The angular velocity sensing mechanism also detects such an oscillation as being caused by the angular motion of the sensor around the angular velocity sensing axis. Specifically, the angular velocity sensing mechanism detects oscillatory movement of the oscillator in error for determining the angular velocity of the sensor when the sensor is not being rotated about the angular velocity sensing axis, but undergoing the acceleration in the second direction.
Conventional angular rate sensors usually have the sensing element installed on a mount base. Use of a rubber plate is proposed which is interposed between the sensor element and the substrate in order to absorb undesirable oscillation of the oscillator arising from application of acceleration. This measure is, however, still insufficient to eliminate the above error in determining the angular velocity of the sensor.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide an oscillatory angular rate sensor designed to minimize an output error caused by application of an undesirable acceleration force to the sensor.
According to one aspect of the invention, there is provided an angular rate sensor unit which comprises: (a) a mount base; and (b) a sensor element installed on the mount base. The sensor element includes generally a substrate, an oscillator, an oscillation exciting mechanism, and an angular velocity sensing mechanism. The oscillator is retained by the substrate through a first support member elastically deformable in a first direction and a second support member elastically deformable in a second direction perpendicular to the first direction. The oscillation exciting mechanism works to excite the oscillator to oscillate in the first direction. The angular velocity sensing mechanism works to produce a sensor output as a function of degree of oscillation of the oscillator in the second direction arising from angular motion the oscillator experiences about an axis extending in a third direction perpendicular to the first and second directions during oscillation of the oscillator in the first direction. A resonant frequency of a structure made up of the mount base and the sensor element in the second direction is set to a value less than or equal to a reciprocal of square root of two times a difference between a resonant frequency of the oscillator in the first direction and a resonant frequency of the oscillator in the second direction.
If the sensor unit experiences an acceleration of a frequency which is identical with the difference xcex94f between the resonance frequencies of the oscillator in the first and second directions, it will result in an increase in degree of oscillation of the oscillator, which is added as a noise to a sensor output. In order to alleviate this problem, the resonant frequency of the structure made up of the mount base and the sensor element in the second direction is set lower than or equal to xc2xdxc2xdxc3x97xcex94f, thereby damping the undesirable oscillation of the structure in the second direction greatly.
In the preferred mode of the invention, the resonant frequency of the structure in the second direction is higher than or equal to a cut-off frequency in angular motion response of the sensor element. Generally, a sensor unit of the type, as discussed herein, is required not to resonate when the sensor unit is rotated at a lower speed, that is, when the angular velocity is lower than a given frequency (i.e., the cut-off frequency). The resonance frequency of the structure in the second direction is, therefore, set higher than the cut-off frequency, thereby achieving desired response of the sensor unit to angular motion.